The present invention relates to a pneumatically operated screw driver, and more particularly, to the screw driver having a mechanism for assisting separation of a screw from a screw band.
A pneumatically operated screw driver uses a screw band to which a plurality of screws are arrayed and held. The screw band is made of a resin material, and each head of the screw can be removed from the screw band by the deformation thereof upon application of shearing force to the screw band.
A conventional electrically operated screw driver shown in FIGS. 13 and 14 includes a main body 1 having a housing 4 and a nose portion 18. The housing 4 has a handle 4a at a rearmost side of the screw driver and houses therein an electric motor 2. The nose portion 18 has an abutment piece 63 at a frontmost side thereof with which a workpiece 42 is abuttable. A plurality of screws 44 are arrayed in line and held with a constant interval by a screw band 43 held at a tip end of the nose portion 18. More specifically, the screw band 43 has one longitudinal side where each head of the screw 44 is exposed, and has another longitudinal side in contactable with a first slide member 32 described later. A drive bit 16 extends through the nose portion 18. The drive bit 16 is movable in its axial direction and rotatable about its axis by the rotation force of the electric motor 2. The tip end of the drive bit 18 has a cruciform shape to engage a cruciform groove formed at the head of the screw 44.
The nose portion 18 has the first slide member 32, a second slide member 33, a casing 34, and springs 38 and 39. The first slide member 32 is positioned at the front end of the nose portion 18, and has a sprocket 49 engageable with the screw band 43. The first slide member 32 has a flat surface 40 where a groove 41 is formed. The first slide member 32 is formed with a circular screw passage (not shown). The second slide member 33 is positioned immediately behind the first slide member 32. The spring 39 is interposed between the casing 34 and the second slide member 33 for urging the second slide member 33 frontwardly. Further, the spring 38 is interposed between the first and second slide members 32 and 33 for urging the first slide member 32 frontwardly. The second slide member 33 has a roller 36 selectively positioned on the flat surface 40 or engageable with the groove 41. The second slide member 33 also has a feed pawl 50 engageable with the sprocket 49 for rotating the sprocket 49 upon relatively frontward movement of the second slide member 33 with respect to the first slide member 32. The second slide member 33 is slidable with respect to an inner surface of the casing 34. In the inner surface of the casing 34, an engagement groove 34a is formed with which the roller 36 can be selectively engaged.
In a non-operation state, the roller 36 is engaged with the engagement groove 34a of the casing 34 and rides on the flat surface 40 of the first slide member 32. Upon frontward movement of the main body 1 with respect to a workpiece 42 after abutting the abutment piece 63 against the workpiece 42, the first slide member 32 is moved into the casing 34 against the biasing force of the spring 38. Therefore, the feed pawl 50 pushes the sprocket 49 to rotate the sprocket 49, so that the leading end screw 44 held by the screw band 43 is fed to the position in front of the drive bit 16 in axial alignment therewith. In this case, the second slide member 33 is maintained at its frontward position by the biasing force of the spring 39 and engagement of the roller 36 with the engagement groove 34a.
On the way to rearward movement of the first slide member 32, the roller 36 which has been riding on the flat surface 40 is then disengaged from the engagement groove 34a and brought into engagement with the groove 41. Thus, the second slide member 33 is released from the casing 34 and engaged with the first slide member 32. Accordingly, the first and second slide members 32 and 33 are moved together into the casing 34. In the rearward movement of the first and second slide members 32 and 33, the drive bit 16 is brought into engagement with the cruciform groove of the head of the screw 44, and the screw band 43 is pressed onto the inner wall of the first slide member 32 in accordance with the rearward movement of the first slide member 32. The screw band 43 is deformed along the circular screw passage formed in the first slide member 32. If the deformation of the screw band 43 exceeds the screw holding force, the screw 44 can be separated from the screw band 43, and passes along the screw passage and screwed into the workpiece 42. If the main body 1 is moved away from the workpiece 42, the first and second slide members 32 and 33 are moved to their original positions by the biasing forces of the springs 38 and 39.
With the above-described conventional arrangement, the screw band 43 is conically deformed for separating the screw 44 from the screw band 43, because the screw band 43 slidingly passes through a circular screw passage formed in the first slide member 32 while being pressed by the head of the screw 44. As a result, relatively large shearing force is required for separating the screw 44 from the screw band 43. To this effect, a sufficiently large scale force is required to manually press the entire screw driver against the workpiece 42 so as to sustain the separation force. This degrades the operability of the screw driver.